When President Obama and the First Lady touched down in South Africa, he was the first man to greet them. The face of the United States, in South Africa, Ambassador Patrick Gaspard.For the past 48 hours, our top diplomat has given Nightline rare access to the whirlwind preparations, as the world pays tribute to Nelson Mandela.His role: to express American grief in this time of mourning. And oh yeah, the monumental, and unprecedented task of welcoming four U.S. presidents and countless dignitaries.Hard to imagine, just 25 years ago, he was actively working to bring down a government in South Africa.Now he's the nation’s official representative here.